N-Phosphonomethyl-glycine, active ingredient of a total herbicide of a wide spectrum may be prepared, for example by the following methods well known in the art:
(a) processes using imino diacetic acid as the starting material (HU-PS No. 165 965, 167 343 and 172 170), PA1 (b) processes employing symmetric N-trisubstituted-triazines (DE-PS No. 2 609 172 and 2 700 017), PA1 (c) Phosphonomethylation of glycine derivatives with chloromethyl-phosphonic acid derivatives (HU-PS No. 174 479).
HU-PS No. 173 170 describes a process in which the product may be obtained by reacting glycine and formaldehyde in an aqueous alkaline medium followed by the reaction of the formed alkali salt of N-hydroxymethyl-glycine with dialkyl phosphite and the hydrolysis of the obtained ester with acid. Though this last process is less complicated then the earlier known processes, when carried out, in large scale it is accompanied by difficulties. As a disadvantage of this last process may be mentioned, that the selectivity of the process is not sufficient and the endproduct is considerably contaminated by glycine, bis-N,N-phosphonomethyl-glycine, etc. and the product must be subjected to further purification in order to get a product of desired purity.
When carrying out the above reaction in large scale the elemental analysis of the product corresponds to the calculated values, but the product contains in fact 26% N,N-bis-phosphonomethyl-glycine and 6% glycine in addition to the desired N-phosphonomethyl-glycine.
The fact that the values correspond to each other can be explained as follows:
Carbon, nitrogen and hydrogen analytical values in the Examples of the mentioned patent specification are substantially the same in the case of this product mixture as in the case of pure N-phosphonomethyl-glycine.
The correct composition of the product mixture has now been determined by liquid chromatography, gas chromatography, and spectroscopy as well as complexometric titrations and analytical methods based upon nitrosation.
In the course of our investigations we observed that dialkyl phosphites rapidly decompose in an aqueous-alkaline medium. Thus, particularly in large scale processes the formation of bis-phosphonomethyl-glycine predominated in aqueous alkaline media.